Pliolophus vulpiceps Owen, 1858

Bronnert, Constance & Métais, Grégoire, 2023, Early Eocene hippomorph perissodactyls (Mammalia) from the Paris Basin, Geodiversitas 45 (9), pp. 277-326 : 282-283

Pliolophus vulpiceps Owen, 1858

Pliolophus vulpiceps Owen, 1858: 54-70 , pls 2-4.

Hyracotherium leporinum – Owen 1865: 340 (partim), pl.10, fig.2. — Lydekker 1886: 11. — Depéret 1901: 200, 201 (partim), pl. 4, fig. 1.

Hyracotherium vulpiceps – Forster-Cooper 1932b: 432-436, figs 2D, 3A, pl. 50, figs 1-3. — Simpson 1952: 195-206, pl. 37, figs a-d. — Hooker 1980: 104, 106, 108, fig. 3.

Hyracotherium sp. – Simpson 1952: pl. 40, fig. a.

Hyracotherium [sic] – Savage et al. 1965: 8-10, figs 3c, e, f. v.

Hyracotherium aff. vulpiceps – Hooker 1980: 106-108, fig. 2; 1989: 83-84, figs 6.3L, 6.4I-K.

HOLOTYPE. — BMHN-44115 , BMNH-44115a, BMNH-M.10567-61, skull and partial skeleton .

MATERIAL. — See Hooker (1980).

TYPE LOCALITY. — Harwich (United-Kingdom), MP8-9.

DISTRIBUTION. — Harwich (United-Kingdom), MP8-9.

DIAGNOSIS (modified from Hooker 1980). — “ Medium-sized species of Pliolophus , length of P2-M3 41 mm in holotype. Upper molars with very sligthly buccally flexed centrocrista and a very faint rib in the mesostylar position, separated from the buccal cingulum. Lower molar cristid obliqua tend to join trigonids equidistant between protoconids and metaconids. M 3 hypoconulid lobe relatively large and with a median main cusp. P3 with large paraconule; distolingually placed protocone; slightly convex mesiolingual margin and little or no postprotocrista. ”

DIFFERENTIAL DIAGNOSIS. — Largest species of Pliolophus . Upper molar centrocrista more flexed than P. barnesi and P. quesnoyensis , and metaconule larger. Differs from P. barnesi and P. vulpiceps by longer mandibular diastemas.

DESCRIPTION

A part of the description is based on the drawings of Owen (1858). The type-specimen was a complete skull with the articulated mandible. This specimen has been splited in several parts, the mandible and the left maxillary were removed, which considerably damaged the teeth. Some other parts of the skull have been lost, such as the basicranium. The complete history of the specimen is reported by Simpson (1952).

Teeth

Three incisors are present on each premaxillary. A 1 cm diastema is present between the third incisor and the canine.

The P1 is two-rooted and possesses a main cusp labially. There is no P1-P2 diastema. The P2 is narrower anteriorly. A cingular bulge is present at the level of the protocone. There is no parastyle.The P3 is nonmolariform and triangular in occlusal view. The paraconule is large and located in the middle of the protoloph. The cingulum is developed and almost continuous, blured close to the protocone. The P4 is nonmolariform and triangular in occlusal view. The parastyle is small and low. The paraconule is large, slightly smaller than the paracone and located in the middle of the protoloph. The preparaconule crista links the preparacrista in the middle. The upper molar parastyle is developed and high. The centrocrista is flexed. The paraconule is large and included within the protoloph. The metaconule is smaller than the metacone. The protoloph links the preparacrista. The metaloph is interrupted at the base of the metacone. The mandibular symphysis is long and narrow (see Owen, 1858). Three incisors are present on each hemimandible. A diastema as long as the canine mesio-distal length is present between the incisors and the canine. A 1 cm post-canine diastema is present. The p1 is simple and large, unicuspid. A short p1-p2 diastema (2 mm) is present. The p2 displays one single cusp, located anteriorly. The p3 possesses a small metaconid closely appressed to the protoconid. The paraconid is absent. The hypoconulid seems developed. The p4 has a low protolophid. The protoconid and the metaconid are about the same height. The cristid obliqua is oriented toward the middle of the protolophid. The entoconid is very small. The lower m1-2 have a twinned metaconid. The cristid obliqua joins the middle of the protolophid at half height. The hypolophid is low, the protolophid is slightly higher. The m3 has a developed hypoconulid, but the lobe is quite short. The hypoconulid is linked to the hypoconid by a postcristid. The cristid obliqua is oriented toward the middle of the protolophid.

Skull

The nasal incisure is interrupted at the level of the canine. It is formed by the premaxillary and the nasal. The anterior border of the orbit is above the M1. The infra-orbital foramen is located at the level of the P2. The sagittal crest is poorly developed. The basicranium and the rostrum have the same length. The horizontal ramus of the mandible is low.

Postcrania

The humeral head is rounded and situated at the end of a short neck. The distal trochlea is shallow. The epicondyles are poorly developed. The supracondylar foramen is large and located in the radial fossa. The greater trochanter of the femur is high. The lesser trochanter is slightly less projected than the third trochanter. The third trochanter is located distally to the distal part of the lesser trochanter. The femoral head possesses a notched fovea capitis. Distally, the lateral condyle is broken.

COMPARISONS

The mandible and the teeth are larger than those of P.quesnoyensis . The hypoconulid-hypoconid junction on m1-2 is the same than the one observed in P. quesnoyensis and some specimens of P. barnesi . It differs from the small hypoconulid closely appressed to the hypolophid in Cymbalophus . The upper molar centrocrista is slightly more flexed than the one of P. barnesi and P. quesnoyensis . The upper molar metaconule is more developed than in the other species of Pliolophus . The paraconule is more developed than in P. quesnoyensis . The p1 is more developed than in P. quesnoyensis .

COMMENT

The few known specimens hinder the study of intraspecific variability in P. vulpiceps . P. vulpiceps displays several more derived features than P. barnesi and P. quesnoyensis , such as a more flexed centrocrista and longer mandibular diastemata.